As laboratory work has increased in fields such as biotechnology, chemistry and medicine with substances having substantial (and in many instances not understood) dangers, there is increasing need to assure that substances do not migrate from the laboratory into the atmosphere. As a result, various containment levels have been established for rooms or other spaces in which experiments on these substances are performed, which standards include biological level 3 and level 4 (BL3, BL4). For each each of these standards, a degree of sealing is required for the room, with a selected low level of leakage being permitted and with required pressure control tolerances.
Similar problems arise for semiconductor and other processes which must be conducted in a "clean room" or aseptic (i.e. class 1000) environment to avoid contaminants. Such clean rooms are typically sealed and positively pressurized to assure that undesired contaminants do not get in.
In most sealed environments, whether they are to be positively or negatively pressurized, air must be exhausted from and supplied to the room in order to remove potentially dangerous elements (exhausted through a fume hood, for example) while maintaining room pressure. Also, supply airflow may be modulated to control room temperature, ventilation (i.e. air changes per hour), and humidity; all of which must be accomplished without compromising room pressurization. However, in order to maintain a prescribed pressure in the sealed space, exhaust and supply flows must be very precisely controlled because even small imbalances between supply and exhaust can cause large errors in room pressurization. For example, a 3% error in flow control may result in errors in room pressure that are orders of magnitude larger. This effect becomes more pronounced as the pressurized environment is sealed more tightly as a result of the fact that: EQU .DELTA.P=(.DELTA.F/K).sup.2
where,
.DELTA.P=room pressure relative to some outside pressure reference (usually atmospheric pressure). PA1 .DELTA.F=air supplied to the room minus air exhausted from the room. This quantity is known as room offset. PA1 K=a coefficient which decreases exponentially as a room is sealed more tightly.
Therefore, the required accuracy on the flow controllers increases exponentially as room permeability or gas leakage is decreased. In some cases, such as with BL4 facilities where an impervious environment is sought, the required tolerance of flow control, using conventional flow sources, approaches zero. As a result, significant difficulty has been experienced in maintaining desired pressure levels in sealed environments.
in addition to being precise, the flow/pressure controller for sealed spaces must be capable of responding quickly to compensate for pressure fluctuations due to the natural characteristics of the surroundings. Here, the volume of the sealed space will largely dictate the natural frequency at which perturbations in room pressure occur. The affinity for pressure oscillations is high in a sealed environment so, regardless of the accuracy of flow control, the pressure control scheme must provide suitable compensation for this characteristic. As volume is decreased, the frequency of natural oscillation (independent of outside influence) increases. Thus, small sealed environments are most difficult to pressurize. The resulting shorter transient response to pressure in smaller spaces makes pressure control by way of conventional systems unreliable, since these devices cannot respond quickly enough to compensate for pressure fluctuations to be realized. The prior art techniques thus tend to be highly unstable.
As a result, significant difficulty has been experienced in maintaining desired pressure levels in sealed environments, particularly in relatively small sealed environments. Some prior art systems have used control loops to control the flow through control elements, such as dampers or valves, in an attempt to independently maintain the supply flow control element and the exhaust flow control element at desired flow levels. Such schemes have had problems since they do not take account of pressure in the room and are therefore unable to maintain a desired pressure. Thus, slight errors in one of the flow control elements, or the controls therefor, can cause errors in room pressure.
An improvement on this is to measure air flow at the output from one of the flow control elements, for example at the exhaust element for the room, and to feed this measured flow back to control the flow through the other element, for example the supply element. This scheme is better, but becomes unstable when there is a spurious change in the output flow. The speed of system operation is normally not rapid enough to compensate for such fluctuations resulting in numerous overshoots and undershoots until a stable flow between the two valves is again restored. The effect of this instability on room pressure can be unpredictable.
A further improvement on such systems is to include a pressure transducer in the room which provides a measurement of room pressure relative to some pressure reference that is compared with a desired setpoint pressure. The resulting pressure differential can be fed back to control changes in operation of the flow control element. However, if the system starts to oscillate, such pressure control may become unreliable.
Another potential problem is that some valves, such as dampers, vary the flow therethrough as a function of the pressure thereacross. Since the supply duct or plenum and the exhaust duct or plenum for a given room may also serve other similar rooms in the same building, pressures in the ducts may vary as a result of what is happening elsewhere in the building, and this in turn will result in a change in flow through the dampers leading to the room being controlled. This coupling of the sealed room to other rooms or areas in a building is undesirable.
To avoid such coupling, a tracking loop may be provided for the damper or other valve which renders the flow through the valve substantially independent of the pressure thereacross. Standard venturi valves, of a type known in the art, have a similar, substantially flat, flow versus pressure characteristic through most of their operating range. However, the pressure in a room will be determined by the point at which the characteristic flow versus pressure curves for the input flow control element and the exhaust flow control element are the same. With flat, pressure independent regions, and particularly where there may be slight ripples in such regions, there are multiple points at which equilibrium for the flow elements occurs, causing the system to oscillate and to be unstable, with room pressure being unpredictable.
A need therefore exists for an improved pressurization control system for sealed spaces which provide only a single pressure at which the flows for the supply and exhaust flow control elements match, while still substantially decoupling the sealed space from the ducts which access the space and thus from surrounding areas of the building containing the space. It is also desirable that such a system permit the two flow control elements to act in tandem without reacting to spurious changes in flow through one of the elements, thus assuring that the system rapidly responds to pressure affecting changes without instability and without overshoots and undershoots. Finally, such system should be capable of damping pressure oscillations or resonance to a level below that at which the system responds to pressure changes so as to prevent such oscillations from adversely affecting system control even for tightly sealed rooms.